In this proposal we describe a set of experiment designed to test how sensory signals are represented at the first synapse of the accessory olfactory system. The accessory olfactory system detects chemical signals pertinent to behavioral responses ranging from aggression to neuroendocrine control of reproduction in animals. Sensory neurons in the nose project to the accessory olfactory bulb, where axons from neurons expressing the same receptor type co-converge on small regions of neuropil called glomeruli. Individual receptor types converge within subsets of glomeruli in a single lamina of the laminar accessory olfactory bulb. Within these glomeruli, sensory neurons synapse on mitral cells, which encode sensory signals and project out of the olfactory bulb. It is unknown whether the response patterns of the sensory neurons and mitral cells have a spatial correlate in the glomeruli. In addition to connections with mitral cells, sensory neurons drive activity in glomerular inhibitory interneurons. GABA(A) and (B) receptors are expressed both pre- and post-synaptically; these inhibitory receptors are candidate effectors for a variety of circuit and sensory processing tasks from scaling and normalization of inputs to the encoding of specific memories for the scent of other animals. However, virtually nothing is known about the degree to which inhibition shapes responses within the glomeruli. For example, it is unknown whether the inhibition is lateral (meaning, it acts between glomeruli corresponding to different receptors), and what the consequences of inhibition are for the output neurons of the bulb, the mitral cells. To examine the specificity of glomerular activation and mechanisms of inhibition, we will exploit the fact that the levels of signaling in sensory neuron terminals within the glomeruli are optically measurable as changes in calcium concentration, thus rendering the entire population response to a stimulus amenable to optical recording. In Aim 1, we will identify the spatial patterns of glomerular activation within the accessory olfactory bulb using sulfated steroids known to drive activity in both sensory neurons and mitral cells. We predict that glomerular activations by different ligands will be functionally separate and have stereotyped spatial distributions. In Aim 2, we will determine the role of inhibitory neurotransmission in shaping the responses of co-activated glomeruli. Populations of glomeruli will be identified by responses to individual sulfated steroids, and then the inhibitory interactions will be measured when the populations are simultaneously activated. The role of both GABA(A) and GABA(B) receptors will be measured, via GABA(A) and (B) antagonism. We predict that GABAergic inhibition of sensory neuron pre-synaptic terminals will occur both within glomeruli and between adjacent co-active glomeruli. If successful, these experiments will provide a mechanistic basis for recent observations such as diminished estrogen sulfate responsiveness of mitral cells relative to sensory neurons and sex-specific odor opponency.